1. Field of the Invention
The present invention relates to a stable polymer composition for peroxide bleaching of a cellulosic fibre material. The stable polymer composition can be used in alkaline peroxide bleaching of mechanical, chemical, chemi-mechanical and de-inked pulps and in deinking of recycled fibers without using silicate as a stabilizer. The present invention also relates to a process for bleaching a cellulosic fibre material with a peroxide compound in an aqueous alkaline medium by using said stable polymer composition.
2. Description of the Related Art
Alkaline silicate solutions normally called water glass have been used in stabilizing hydrogen peroxide solutions, which are used in alkaline peroxide bleaching of mechanical pulps. Nowadays it is more common not to make a pre-prepared bleaching liquor, but water glass is fed separately, when mechanical pulps are bleached with peroxide and alkali.
Water glass is used alone or together with peroxide in de-inking of recovered papers. Sometimes the de-inked pulp is also bleached with alkaline peroxide.
The use of water glass in alkaline peroxide bleaching of chemical pulps has been published, but the method cannot be utilized in full scale, since the silicate can cause very severe precipitation problems. Another disadvantage with water glass is that when the bleaching liquors are recycled and ultimately fed into the recovery boiler, where the so-called black liquor from the cooking process after concentration is burned, the silicate will cause severe scaling and thus decrease the heat transfer in the recovery boiler, which in worst case can cause an explosion of the recovery boiler. Further the use of the silicate can cause highly dusting.
If the silicates, e.g., in form of the water carry-over, will enter the paper making process, they will disturb the papermaking process, e.g., by precipitating on hot surface, causing holes in the paper reel etc.
It is known that hydrogen peroxide will decompose very rapidly in an alkaline milieu in the presence of heavy metal (transition metal) ions. The most abundant of these ions in pulps are iron and manganese. The copper ion is also very detrimental for alkaline hydrogen peroxide, but normally it can enter the process only via used process waters.
It is also known that iron will start to precipitate already below pH 7, first in colloidal form. The formed iron hydroxides, oxyhydroxides etc are much more catalytically active than iron ions. Also manganese can, at least partly, be in precipitated form, but it has been shown that in the presence of hydrogen peroxide, manganese should be in dissolved form.
The theory of the function of water glass varies, but one theory is that water glass will deactivate the catalytic surface of iron and other heavy metal ion “precipitates”. In order to avoid the detrimental effect of manganese ions, a chelating agent is often introduced into the bleaching process or the pulp is pretreated with a chelating agent. The most common chelating agents are EDTA and DTPA, which belong to the group of polyaminopolycarboxylates. The corresponding phosphonates, EDTMPA and DTPMPA can be also used, but they are much more expensive than the polyaminopolycarboxylates. They have also the disadvantage that they contain phosphorus, which is not a wanted component, when the environmental regulations are becoming stricter and stricter.
According to the above there is a need to replace water glass (silicates) in alkaline peroxide process and in pulping processes, which use water glass, e.g., in de-inking of recovered paper. There have been suggestions to use phosphonates, but they should be used in quite high dosages and the phosphorus problem in the waste waters would still remain. Since the common phosphonates are non-biodegradable, there has been much studies about they adverse effect on mobilizing heavy metals, e.g. from sediments in waterways.
One solution to stabilize alkaline hydrogen peroxide solutions or to avoid water glass is based on the use of poly-alfa-hydroxyacrylic acid (PHAA). Typically this substance is used as a sodium salt of poly-alfa-hydroxyacrylic acid made from the corresponding lactone (an internal ester) by alkaline treatment (such as NaOH). Thus, the sodium salt of poly-alfa-hydroxyacrylic acid typically appears only as an aqueous alkaline solution. The lactone is not soluble in water. When the name PHAA is mentioned, it normally does not refer to the free acid, but to a salt thereof.
U.S. Pat. No. 4,363,699 describes a process for stabilizing alkaline solutions of peroxidic compounds used for bleaching wherein an alkali salt of a poly-alfa-hydroxyacrylic acid is added to the solution as a stabilizer.
EP 0 842 321 B1 describes a method of stabilizing alkaline bleaching liquors containing oxygen and/or hydrogen peroxide and three different types of chelating agents. The first one can be a phosphonate or an aminopolycarboxylic acid, and the second one can for example be poly-alfa-hydroxyacrylic acid used as a salt. The third one is a protein derivative.
EP 0 814 193 discloses a silicate-free stabilizing agent for peroxide-bleaching procedures, comprising a) a first component selected from homopolymers of alpha-hydroxyacrylic acid and copolymers of alpha-hydroxyacrylic acid with other comonomers, and water soluble salts and polylactones of the mentioned homo- or copolymers in combination with b) a second component selected from homopolymers and copolymers of acrylic acid, methacrylic acid and maleic acid, copolymers of at least one of the above-mentioned acids with other comonomers and salts of above-mentioned homo- and copolymers, and c) a third component selected from the common chelating acids DTPA and TTHA and salts thereof, and optionally d) a fourth component selected from water-soluble magnesium salts. The poly-alfa-hydroxyacrylic acid is used as its sodium salt.
EP 0 814 193 also describes a method of bleaching a fibre material comprising pretreating the fibre material with an aqueous solution of the above mentioned stabilizing agent typically for a period of one hour and at a pH of between 6 and 11, preferably between 7 and 10.5, and then bleaching the pretreated fibre material with an aqueous solution of a peroxide bleaching agent The pretreated fibre material is preferably washed before the bleaching.
EP 0 814 193 additionally describes a method of bleaching a fibre material comprising bleaching the fibre material with an aqueous alkaline peroxide bleaching liquor comprising the above mentioned stabilizing agent.
DE 3423452 discloses a solution for avoiding the use of water glass comprising a stabilizing mixture of a poly-alpha-hydroxyacrylic acid (PHAA) and a water soluble homopolymer of acrylic or methacrylic acid or a copolymer of acrylic acid and/or maleic acid. The salts, especially the sodium salt of PHAA and the sodium salt of the polycarboxylate polymer are mixed together. Chelating agents can be added to the mixture and thus an improved stabilizer mixture for bleaching processes is obtained. According to DE 3423452 the bleaching process is carried out by using an alkaline peroxide bleaching liquor containing said stabilizing mixture.
Since it is known that hydrogen peroxide will very easily decompose in alkaline solutions, a ready made bleaching mixture is not a very feasible choice when bleaching pulp with hydrogen peroxide in an alkaline milieu.
In EP 0 801 169 poly-alpha-hydroxyacrylic acid or the corresponding salts or polylactone is used in the alkaline peroxide bleaching of a chemical pulp at a temperature of above 100° C. and in the presence of a compound chosen from potassium hydroxide, sodium hydroxide and alkali metal or alkaline-earth metal carbonates.
It has been found that the alkaline earth metal ions can be very detrimental for PHAA. When paper is made from a mechanical pulp, is it very common to feed the so-called white water from the paper making process to the pulping process. The white water contains very often high contents of calcium ions when calcium containing coating pigments or fillers have been used in the paper making process. If a chemical pulp is bleached, the pulp will contain very high amounts of calcium ions, which are released to certain extent in each bleaching stages. Also the recycling of the different waste water streams will contribute the amount of alkaline earth metal ions.
If PHAA is used alone it will bind calcium and magnesium ions and render PHAA less effective due to the content of alkaline earth metal ions. The reason of this is not known, since e.g. the sodium salt of PHAA and a sodium salt of a conventional polyacrylic acid have almost the same calcium binding ability in the presence of magnesium at neutral pH, while the degree of binding of magnesium is much weaker but stronger for the polyacrylate than for the sodium salt of PHAA. [T. Tamura et al., Polymer International 46 (1998), 353-356].
According to the present invention it was surprisingly found that by using a polycarboxylate made by homopolymerization of acrylic or methacrylic acid or more preferably by copolymerization of acrylic and/or methacrylic acid with an unsaturated dicarboxylic acid, such as maleic acid, and adding this polymer to the polylactone, the efficiency of the polylactone, probably in the form of acidic PHAA could be maintained. This means that a smaller amount of the effective and expensive component, i.e. PHAA, could be used, when the product made from the polylactone and the polycarboxylate was used than when using PHAA made from the polylactone alone.
The prior art teaches that when PHAA or the corresponding polylactone is used together with a polycarboxylate made, e.g., by homopolymerization of acrylic acid or made by copolymerization of acrylic acid with an unsaturated dicarboxylic acid, such as e.g. maleic acid, the acids are normally provided in form of salts, e.g., as alkali salts. This means that, if the polymerization is carried with acidic monomers, they have to be treated with e.g. alkali in order to get sufficient water solubility. This will add an extra process step.
PHAA as a sodium salt and the polycarboxylate polymer as a sodium salt can be used in the bleaching process separately, but this will require two separate pumping and controlling systems. The two polymers can also be made as ready made mixture, but the polycarboxylate will be in the form as sodium salt.